1. Field of the Invention
The present invention relates to a power supply apparatus such as a switching regulator and, more particularly, to a soft start technology associated with the apparatus.
2. Description of the Related Art
FIG. 12 is a block diagram of a power supply apparatus according to the related art. As illustrated, the power supply apparatus is usually provided with soft start circuits 101a-101c for the purpose of preventing destruction of switching transistors due to a rush current occurring at start-up and preventing overshoot in the waveform of an output voltage as it rises.
The soft start circuits 101a-101c are configured to gradually increase (soft start) an output voltage by increasing a conduction period (on duty) of the switching transistors over a predetermined period of time since start-up. For example, on-duty of the switching transistors is controlled so that the output voltage is increased in accordance with the waveform of the charged voltage of capacitors 104a-104c of a time-constant circuit.
However, when a plurality of loads (117a-117c) requiring different supply voltages (Vo1-Vo3) are provided in a single apparatus, there may arise a need to provide a plurality of power supply circuits 105a-105c in the apparatus.
In this case, the power supply apparatus is often subject to on and off control by a host 111 after a sufficient period of time has elapsed since power from a power supply voltage Vcc indicated in the drawing is turned on (hereinafter, referred to as power-on), for the purpose of power saving. Accordingly, such a multi-channel power supply apparatus needs the soft start circuits 101a-101c and the external capacitors 104a-104c for respective channels, thwarting efforts reduce the number of components.
“Example of applied circuits” section in non-patent document 1 describes “capacitors SS1, SS2, SS3” which correspond to the external capacitors described above.
[Non-Patent Document]
Manual of “HDD system power supply BD9786KN”, January 2003, Rohm Corporation